As is known in the art, when a MMIC (a chip) is coupled to an input circuit board using a bond wire, such as shown in FIGS. 1A–1B, the MMIC is formed with a capacitive input impedance (FIG. 1C) in order to compensate for the inductance of the bond wire. The capacitance of the capacitive input impedance is selected in accordance with the inductance of the bond wire to create a series resonant L-C circuit with the result that real, typically 50 ohm, input impedance is presented to the source. Likewise, the MMIC is formed with a capacitive output impedance (FIG. 1C) to compensate for the inductance of the bond wire used to couple the output of the MMIC to the load presented by the circuit board. The capacitance of the capacitive output impedance is, as with the input impedance, selected in accordance with the inductance of the bond wire to create a series resonant L-C circuit with the result that real, typically 50 ohm, output impedance is presented to, in this case, the load.
As is also known in the art, during manufacturing of the MMIC, and before connected of the MMIC to the board or load, it is frequently necessary to test the MMIC. That is, high frequency Microwave Monolithic Integrated Circuits (MMIC's) must be radio frequency (rf) tested “on-wafer” prior to separating the die (i.e., chip) for bonding into modules, packages or circuit boards. Both large signal and small signal testing are carried out using commercially available probes. A typical test probe (FIG. 1D) has a 50 ohm impedance at the tips of the probes. Further, during the testing, since the circuit board is absent, the bond wire is also absent, as shown in FIGS. 1D and 1E. Thus, during the on-wafer testing, there is an impedance mismatch between the test probe and the MMIC being tested (i.e., the inductance of the bond wire is not present during the pre-die testing yet the MMIC still has the capacitive input impedance and capacitive output impedance used to compensate for the anticipated bond wire inductance).
More particularly, typical MMIC designs are aimed at nominal 50 ohm input and output characteristic impedance. This is achieved with MMIC microstrip by having a center conductor separated from the ground plane by the semiconductor dielectric. In a fixture, the center conductor is bonded to a pad on the mounting substrate. As shown in FIGS. 1A–1C, the bond wire and associated bonding pad form an inductance that can be significant at high frequency. In particular, at 44 GHz, the inductance of this bond wire must be accounted for in the MMIC design. FIG. 1C shows how an equivalent capacitive reactance is incorporated in the MMIC input and output matching network to cancel the bond wire inductance. To utilize probes for in-process probe test, the wafer is processed through thinning (typically to 50 microns for 44 GHz). Via holes minimally close to top ground pads pass through the thinned wafer. This results in a Ground-Signal-Ground (GSG) configuration as shown in FIG. 1D. The configuration provides a transition from 50 ohm co-planar probes to the microstrip on the MMIC. The line where the GSG probes contacts the MMIC, is intended to be 50 ohms. In the conventional probe pad design, there is no provision to correct for the bond wire inductance of the fixtured MMIC. Therefore, the MMIC, which still has the compensation capacitance for the absent bond wire inductance, is improperly tuned during probe test.